Integrated circuit (IC) fabrication is based upon the formation of precise patterns upon the surface of substrates, typically silicon wafers, using photolithography. The formation of precise photolithographic patterns is dependent upon the application of uniform films of photosensitive materials, also known as photoresist. Photoresist is applied as a light sensitive polymer coating to protect selected areas on a substrate during subsequent chemical treatments. Photoresist can be either negative-acting or positive-acting. With negative-acting photoresist, the coating remains in the light-struck areas. Positive-acting photoresist is the converse. Regardless of the type of photoresist used in the IC fabrication process, a uniform coating of the photoresist is very important because the thickness of this photoreactive layer can impact subsequent processing steps.
Several dispensing methods have been employed to apply liquid photoresist onto wafer substrates. Typically, spinning wafers are flooded with photoresist, dispensed from nozzles in a wafer track system. These dispense nozzles all have orifices with circular cross sections. The wafers are then subjected to high acceleration to evenly distribute the photoresist over the wafer surfaces.
In one prior art method called the center dynamic dispense, the dispense nozzle is held above the spin axis of the wafer substrate, and photoresist is dispensed from the nozzle onto the spinning substrate. Once the wafer substrate is flooded with the photoresist, it is rapidly accelerated to a predetermined spin speed to spread the photoresist into a uniform film at the desired thickness. During this high acceleration, about 96% of the photoresist is normally flung off the wafer.
In another prior art method called the center static dispense, the wafer substrate is held motionless while photoresist is dispensed at the center of the substrate. The substrate is then subjected to a high acceleration to cause the photoresist to spread to a uniform film at the desired thickness. Excess photoresist is again flung off the wafer.
In yet another method, the dispense nozzle is scanned across the spinning substrate while dispensing photoresist. The substrate is flooded with photoresist and then subjected to high acceleration to a predetermined spin speed to form a film of uniform thickness at the desired thickness. This method is called the reverse/forward radial dynamic dispense depending on the direction of nozzle translation across the substrate.
All of these prior art methods depend upon the application of relatively large volumes of photoresist in order to achieve films of uniform thickness. Radial dynamic dispensing helps to spread material across the substrate somewhat. As presently practiced, however, the fluid flow onto the substrate is not smooth; the uniformity of the fluid spread the during dispense is poor; and relatively large excess volumes of fluid are required to achieve acceptable film thickness uniformities. In addition to these disadvantages, the cost of the photoresist material has greatly increased for new generation deep-ultraviolet (DUV) technology for finer pattern feature dimensions. To this substantially increased material cost must be added the cost of hazardous waste material disposal.
Hence, a need exists for a nozzle and a method for dispensing photoresist that delivers a uniform layer of photoresist while reducing waste.